The present invention concerns a composite doctor arrangement which is intended for a web forming machine or web handling machine and which has as components a beam structure, a mounting piece fastened to it, and a blade holder fastened to the mounting piece, with a blade included in the doctor arrangement fastened to the blade holder.
Doctor arrangements where the load-bearing element is a beam structure are used in web forming machines, such as in paper, board and tissue machines, and in web handling machines, such as in printing machines. The beam structure is usually supported on the web forming machine only by its ends. Beam lengths are currently up to more than 10 meters. Moreover, the speeds of web forming machines have increased clearly. When the beam structures are manufactured conventionally from metal, they become large and heavy. Hence, they require much installation space, and their own weight alone makes them deflect clearly. The said beam structures are also sensitive to vibration, which complicates the operation of the doctor arrangement.
In order to reduce weight and vibration and to speed up manufacture, beam structures are presently manufactured from materials lighter than metal. In this way, it is also possible to achieve increasingly light-weight but stiffer beam structures which feature better vibration properties than before. However, the composite construction calls for the use of expensive raw materials. Furthermore, it is problematic to join other parts included in the doctor arrangement to the beam structure. Especially the mounting piece intended for the blade holder must be fastened stationarily to the beam structure either with glue or a screw connection. In this case, for example the thermal expansion coefficient of the length of the mounting piece must be very close to that of the beam structure.
In practice, the manufacture of a sufficiently rigid beam structure requires the use of reinforcements such as carbon fiber reinforcements. In this case, the fixed mounting piece must also have corresponding reinforcements, for example, to avoid torsion resulting from differences between the ambient temperature and the temperature of the beam structure. Moreover, the mounting piece becomes large, which increases the manufacturing costs and complicates the placement of the mounting piece in the doctor arrangement. Further problems are caused by the fastening of commercially available blade holders to the composite mounting piece by means of screws. The blade holder is normally made of metal, or at least it has properties which differ from those of the mounting piece. This causes torsion and tension between the parts primarily due to differences in their thermal expansion coefficients. In other words, the manufacture of the doctor arrangement involves several limitations and problems, which add to the manufacturing costs and complicate the use of the doctor arrangement.